Electroplating tool with feedback of metal thickness distribution and correction

ABSTRACT

An electroplating reactor includes an electro-plating solution in a bath, a ring cathode in the bath and located to contact a workpiece such that only the front side of the workpiece is immersed in the solution, plural anodes immersed in the bath below the ring cathode, and plural anode voltage sources coupled to the plural anodes; plural thickness sensors at spatially separate locations on the back side of the workpiece with feedback control to the anode voltage sources.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Application Ser. No.61/834,353, filed Jun. 12, 2013 entitled NON-CONTACT SHEET RESISTANCEMEASUREMENT OF BARRIER AND/OR SEED LAYERS PRIOR TO ELECTROPLATING, byAbraham Ravid, et al., which is herein incorporated by reference.

BACKGROUND

1. Technical Field

The disclosure is related to processes and apparatus for formingconductive films on a workpiece such as a semiconductor wafer, and inparticular for measuring and controlling uniformity in such films.

2. Background Discussion

Semiconductor integrated circuits have conductive structures defined bytrenches filled with a metal such as copper deposited on a barrier layerformed on or over the semiconductor or dielectric substrate. Coppermigration into the semiconductor or dielectric substrate is prevented byforming the barrier layer of a suitable material such as tantalumnitride or titanium nitride, for example. Prior to filling the trenchwith copper, a thin copper seed layer is grown on the barrier layer byphysical vapor deposition. Thereafter, a thick copper layer is depositedover the copper seed layer by electroplating, the thick copper layerbeing sufficient to fill the trench.

One problem with such processes is that non-uniformities in the spatialdistribution of sheet resistance of the barrier layer and/or of the seedlayer lead to corresponding non-uniformities in the spatial distributionof thickness in the copper layer formed by electroplating. Typically,sheet resistance in the underlying seed/barrier layer affects theelectro-plating deposition rate and surface compliance.

A related problem is that thickness sensors, such as eddy current(magnetic field loss) measurement sensors are employed to measurethickness, but typically only after completion of the electroplatingprocess forming the thick copper layer. Generally, eddy current sensorsare not suitable for reliable and sufficiently accurate thicknessmeasurements of thin layer such as the barrier and seed layers (whosecombined thickness is typically on the order of only 50-200 Angstroms).As a result, non-uniformities in copper layer thickness distribution (orelectrical properties distribution) are not discovered until later, andthe affected product wafers must be discarded. Any remedial measures aretaken with reference to the next batch of wafers in the best case.

Another problem is that the electroplating process itself can introducenon-uniformities in thickness distribution. Such non-uniformities aregenerally not discovered until completion of the electroplating processor at a later process step.

A related problem is that a final overall thickness distributionmeasurement is required upon completion of the electroplating process.Typically, such a measurement requires an expensive measurement tool,such as an opaque film metrology device employing optical measurementsof elastic deformations stimulated by ultrasonic energy. A less costlyalternative, such as an eddy current sensor, involves a sloweracquisition of thickness distribution requiring discrete measurements atmany locations across the wafer surface, using a move-stop-acquire-movesequence. In certain Cu plating processes, there are process failuresthat exhibit small-area (<10 mm×10 mm) thickness non-uniformity anywherenear the edge of a 300 mm wafer. Detecting such failures requires a highdensity of measurement locations (>100 sites) along the perimeter of thewafer, decreasing throughput significantly.

SUMMARY

An electroplating reactor comprises: an electroplating bath and anelectro-plating solution in the bath and having a liquid top surface; aring cathode in the bath and located to contact a workpiece such that afront side of the workpiece is immersed in the solution below the liquidtop surface and a back side of the workpiece is above the liquid topsurface; plural anodes immersed in the bath below the ring cathode, andplural anode voltage sources coupled to the plural anodes; pluralthickness sensors at spatially separate locations on the back side ofthe workpiece; and a computer coupled to outputs from the pluralthickness sensors and coupled to the plural anode voltage sources.

In one embodiment, the computer is programmed to infer from the outputsof the sensors a spatial distribution of thickness on the front side ofthe workpiece, and to control a distribution of voltages from the anodevoltage supplies to compensate for non-uniformity in the spatialdistribution.

The plural thickness sensors may be located at different radiallocations and/or may be arranged in a two-dimensional array. In oneembodiment, the anodes may be provided as a two-dimensional array ofdiscrete anodes or as concentric rings.

In one embodiment, an eddy current sensor comprises a planar inductorformed in a thin planar layer comprising plural co-planar conductorloops connected in series and distributed in a plane, an RF oscillatorcoupled to the inductor and an impedance-sensing circuit coupled to theplanar inductor and having an output corresponding to the output of theone eddy current sensor. The conductor loops may comprise a singleelongate conductor, the planar inductor further comprising a dielectricsupport layer underlying the thin planar layer. The plane of theconductor loops may face a plane of the back side of the workpiece. Inone embodiment, the conductor loops define a serpentine shape. In arelated embodiment, each conductor loop is shaped as a rectangular loophaving three straight sides and one open side. In an embodiment, theends of the elongate conductor are adjacent one another and the pluralconductor loops provide a closed path of the plural conductive loopscorresponding to a complete circuit.

In accordance with another aspect, a system for electroplating aworkpiece having a previously deposited metal film, includes: anelectroplating bath and an electroplating solution in the bath andhaving a liquid top surface; a ring cathode in the bath and located tocontact a workpiece such that a front side of the workpiece is immersedin the solution below the liquid top surface and a back side of theworkpiece is above the liquid top surface; plural anodes immersed in thebath below the ring cathode, and plural anode voltage sources coupled tothe plural anodes; plural thickness sensors at spatially separatelocations on the back side of the workpiece; and a computer coupled tooutputs from the plural thickness sensors and coupled to the pluralanode voltage sources; a beam resistance measurement tool comprising anenergy beam source directed to a workpiece to be measured, an electricalsensor coupled to the workpiece to be measured and a second computerprogrammed to infer a spatial distribution of an electrical parameterfrom observations of successive responses of the sensor for differentcurrent paths in the metal film, the second computer coupled to thefirst computer.

In one embodiment, the first computer is programmed to adjust an initialdistribution of the voltages of the anode voltage supplies in accordancewith the spatial distribution of the electrical parameter. In oneembodiment, the first computer is programmed to adjust the initialdistribution of the voltages of the anode voltage supplies to compensatefor non-uniformity in the spatial distribution of the electricalparameter.

In accordance with a yet further aspect, a method of depositing a metallayer on a workpiece by electroplating, includes submersing a workpiecein a bath containing an electrolyte solution having a liquid top surfaceso as to immerse a front side of the workpiece in the solution and leavea back side of the workpiece above the liquid top surface; contactingthe front side of the workpiece with a cathode ring; providing pluralseparate anodes in the bath and respective anode voltage sources coupledto the anodes; affixing plural thickness sensors to the back side of theworkpiece at different locations; inferring from outputs of thethickness sensors a spatial distribution of thickness on the front sideof the workpiece; and adjusting a distribution of voltages among theanode voltage sources to compensate for non-uniformity in the spatialdistribution.

In one embodiment, the anodes are disposed at respective radiallocations, and wherein the adjusting comprises one of: (a) providing acenter-high or edge-low distribution of the anode voltages whenever thespatial distribution is center-low or edge-high, (b) providing acenter-low or edge-high distribution of the anode voltages whenever thespatial distribution is center-high or edge-low.

In one embodiment, the workpiece comprises a previously deposited metalfilm on the front side, the method preceded by: observing responses toan energy beam of an electrical parameter along different current pathsin the metal film, and constructing a model of a spatial profile of theelectrical parameter from the responses; and adjusting a distribution ofanode voltages among the separate anodes to compensate for anon-uniformity in the spatial profile of the electrical parameter.

The energy beam may be one of an electron beam or a light beam, and theelectrical parameter is related to sheet resistivity.

In one embodiment, the observing comprises: (a) providing an electricalreturn contact at a contact location on the metal film; (b) directing anenergy beam to a selected beam impact location on the metal film, thebeam impact location and the electrical return contact defining a pathbetween them; (c) observing behavior of the electrical parameter at thecontact location; (d) inferring from the behavior a value of theelectrical parameter for the path; and (e) moving the contact locationto successive contact locations and moving the beam impact location tosuccessive beam impact locations to define different paths, andrepeating (c) and (d) for each one of the paths.

In one embodiment, constructing a model is carried out by inferring aspatial distribution of the electrical parameter in the metal film bycorrelating successive values of the electrical parameter with thesuccessive paths.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the exemplary embodiments of the presentinvention are attained can be understood in detail, a more particulardescription of the invention, briefly summarized above, may be had byreference to the embodiments thereof which are illustrated in theappended drawings. It is to be appreciated that certain well knownprocesses are not discussed herein in order to not obscure theinvention.

FIG. 1A includes a block diagram depicting an energy beam resistivitymeasurement tool in accordance with an embodiment.

FIG. 1B includes an elevational view corresponding to FIG. 1A.

FIG. 1C is an enlarged elevational view of a portion of the tool of FIG.1B.

FIG. 1D is a plan view of a workpiece and points of contact by a contactring in the embodiment of FIGS. 1A through 1C.

FIGS. 2A and 2B are contemporaneous time domain waveforms depicting,respectively, a single energy beam pulse and a corresponding currentsensor response in the embodiment of FIGS. 1A through 1C.

FIGS. 2C, 2D and 2E are contemporaneous time domain waveforms of,respectively, a succession of energy beam pulses, a correspondingcurrent sensor output and an angular position of a currently enabledcontact rod in the embodiment of FIGS. 1A through 1C.

FIGS. 3A, 3B and 3C depict an alternative embodiment employing a lightbeam source, of which FIG. 3A includes a block diagram, FIG. 3B is anelevational view corresponding to FIG. 3A and FIG. 3C is a plan view ofan anode electrode of the embodiment of FIG. 3A.

FIG. 4 is a schematic block diagram of an integrated system includingthe energy beam resistivity measurement tool of FIGS. 1A through 1C andan electroplating reactor.

FIG. 5A includes an orthographic view depicting one embodiment of anelectroplating reactor in the integrated system of FIG. 4.

FIG. 5B is an elevational view of a portion of the embodiment of FIG.5A.

FIG. 6A includes an elevational view depicting one embodiment of anelectroplating reactor having internal sensors and feedback control.

FIGS. 6B-6J depict aspects of an eddy current sensor having a planarinductor adapted for use in the embodiment of FIG. 6A, of which FIGS.6B, 6C and 6D are respective views of an embodiment of the planarinductor, FIGS. 6E and 6F are different views of the eddy current sensorincluding a planar inductor, FIG. 6G depicts a modification of theembodiment of FIG. 6E including ferrite cores, FIG. 6H depicts amodification having an integral ferrite core layer, FIG. 6I is across-sectional elevational view taken along lines 61-61 of FIG. 6H, andFIG. 6J depicts an embodiment having a rectangular spiral.

FIG. 7A includes a plan view of one embodiment corresponding to FIG. 6A.

FIG. 7B includes a plan view of another embodiment corresponding to FIG.6A.

FIG. 8 includes a block diagram of a high speed thickness measurementtool in accordance with an embodiment.

FIG. 8A includes a block diagram of a high speed thickness measurementtool in accordance with a further embodiment.

FIG. 9 is a plan view corresponding to FIG. 8.

FIG. 10 is a graph representing measurement data obtained in theembodiment of FIG. 8.

FIG. 11 illustrates an eddy current sensor adapted for use in theembodiments of FIGS. 8 and 8A and having a shaped measuring zone such asan arcuate shape.

FIGS. 12, 13 and 14 are plan views of alternative embodimentscorresponding to FIG. 8.

FIG. 15 is a schematic block diagram of an integrated system includingthe embodiments of FIGS. 1A, 6 and 8.

FIG. 16 is a sequential block diagram of a process performed in thesystem of FIG. 15.

FIGS. 17A through 17D are sequential diagrams depicting changes in thinfilm structure on a workpiece during corresponding steps in the processof FIG. 16.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements and features of oneembodiment may be beneficially incorporated in other embodiments withoutfurther recitation. It is to be noted, however, that the appendeddrawings illustrate only exemplary embodiments of this invention and aretherefore not to be considered limiting of its scope, for the inventionmay admit to other equally effective embodiments.

DETAILED DESCRIPTION Contactless Sheet Resistance DistributionMeasurement:

FIGS. 1A, 1B and 1C depict a measurement tool employing an electron beamfor measuring a material characteristic (such as sheet resistance) inbarrier and seed layers prior to electroplating. Referring to FIGS.1A-1C, a workpiece such as a semiconductor wafer 100 has a top surfaceincluding a main region 110 in which are formed integrated circuitstructures, and a peripheral contact belt 115 as an annular zone at thewafer periphery surrounding the main region 110 and having a smoothtopology devoid of any circuit structures. A metal layer 105 covers themain region 110 and the peripheral contact belt 115. Typically, themetal layer 105 can include a barrier layer of tantalum nitride ortitanium nitride and a copper seed layer grown by physical vapordeposition on the barrier layer. The wafer 100 depicted in FIGS. 1A-1Chas not undergone the electroplating process, and therefore the metallayer 105 is thin, on the order of 50-200 Angstroms, the combinedthickness of the barrier and seed layers. The measurement tool of FIGS.1A-1C facilitates a measurement of the spatial distribution of sheetresistance of the metal layer 105 across the top surface of the wafer100. Because the metal layer 105 is very thin, such a measurement may beimpractical if carried out using an eddy current sensor.

In one mode, the measurement tool of FIGS. 1A-1C employs an electronbeam. A material characteristic (such as sheet resistance for example)is measured by observing an electrical parameter. In one embodiment, theelectron beam is a pulsed beam, and the observed electrical parameter isthe decay time of a current generated in the metal layer 105 during theoff time between successive pulses of the electron beam. In anothermode, the tool of FIGS. 1A-1C employs a continuous electron beam source,and the observed electrical parameter is the amplitude of the electricalresponse (e.g., voltage or current) in the wafer. As will be describedbelow, different measurements may be made along different current pathsin the metal layer 105.

Electrical contact is made to the peripheral contact belt 115 on theperiphery of the wafer 100 in any suitable manner. For example, onemanner of making such electrical connection is to hold a contact ring130 on the peripheral contact belt 115 of the wafer 100. The contactring 130 is an annulus coupled to an electrical reference potential(e.g., ground) through an electrical sensor 135, which may be a voltagesensor or a current sensor, for example. A beam source 140 is suspendedover the top surface of the wafer 100, and produces an electron beam 142that strikes the wafer 100 at a beam impact point 145. The wafer 100acts as an anode and may be grounded, collecting electrons from theelectron beam 142. Actuators including an X-axis stage 150 and a Y-axisstage 155 control a two-dimensional location of the beam source 140 in aplane parallel with the wafer top surface. In one embodiment, thediameter of the beam source 140 is sufficiently small to avoidinterference with movement of the beam source 140 by the X-axis stage150 and the Y-axis stage 155, permitting movement of the beam impactpoint 145 across the diameter of the wafer 100. In one embodiment, thecontact ring 130 contacts the peripheral contact belt 115 throughdiscrete electrically separate pointed contact rods 160 provided on thecontact ring 130 and distributed circumferentially on the contact ring130. As shown in FIG. 1C, each contact rod 160 has an upper portion 160a above the contact ring 130 and a lower portion 160 b contacting thewafer 100 below the contact ring 130. Respective switches 165 areconnected in series between the respective contact rods 160 and theelectrical sensor 135. A controller 170 governs the switches 165 andgoverns the pulsing of the beam source 140 in a pulsed mode ofoperation. In one embodiment, the wafer 100 and the electron beam 142are enclosed in a vacuum enclosure 172 evacuated by a vacuum pump 173.

The electron beam 142 generates an electrical current in the metal layer105 that flows from the point of the beam impact point 145 to groundthrough the contact ring 130 and the electrical sensor 135.

If the beam source 140 is operated in a pulsed mode, then at the end ofeach beam pulse, this current decays at a rate determined (at least inpart) by the resistance and capacitance in the path followed by thecurrent. If the decay is an exponential function, then the time in whichthe current decays from an initial magnitude to a fraction 1/e of thatinitial magnitude is function of the resistance R, inductance L and thecapacitance C of the path followed by the current. The effect of theinductance L is typically negligible, and the decay time is principallydetermined by R and C, and may be referred to as the RC decay time. Acomputer 180 coupled to the electrical sensor 135 deduces thetime-domain waveform of the output of the electrical sensor 135 (e.g.,the RC decay time), which is the observed electrical parameter. Fromthis electrical parameter, the computer 180 computes the value of amaterial characteristic (e.g. sheet resistance) of the current path inthe metal layer 105, in accordance with well-known principles.

In one embodiment, the controller 170, governed by the computer 180,enables a single one of the switches 165 during (and slightly after)each pulse, and may enable different switches 165 during successivepulses. The controller 170 also changes the location of the impact point145 of each pulse by controlling the X- and Y-axis stages 150, 155.Thus, the current path for each pulse is precisely established betweenan X-Y location of the impact point 145 and one of the contact rods 160corresponding to the one switch enabled during the current pulse. Overthe course of numerous pulses of the pulsed electron beam 142, the X-Ylocation of the beam impact point 145 and the chosen contact rod 160 maybe changed in any sequence determined by the computer 180. By thuschanging the electrical paths for successive pulses, the computer 180can deduce a spatial distribution of the electrical characteristic(e.g., sheet resistance) over a number of pulses.

In one embodiment, the electron beam source is operated in the CW(continuous) mode, and the observed electrical parameter is the rawmagnitude of the electrical response (e.g., a D.C. voltage or current)sampled at the electrical sensor 135. The computer 180 infers from thisraw magnitude a value of the material characteristic of interest (e.g.,sheet resistance). This inference may be made by comparing the rawmagnitude with results previously obtained in calibration samples whosematerial characteristic (e.g., sheet resistance) is known. A tabulationof the results of many such calibration samples covering a desired rangeof values may be accessed by (or stored in) the computer 180, for rapidtranslation of measurements on a production wafer.

In one embodiment of this mode, the beam impact point 145 and theselection of one of the contact rods 160 are changed to establishdifferent current paths, and respective measurements of the electricalparameter are obtained for respective paths.

For an embodiment in which the beam source 140 is a pulsed beam source,FIG. 2A depicts a single pulse by the pulsed beam source 140, while FIG.2B depicts a time domain waveform of current in the metal layer 105 ofFIG. 1A generated by that pulse, as sensed by the electrical sensor 135.

FIG. 2C depicts a succession of pulses from the beam source 140, FIG. 2Ddepicts a succession of responsive currents generated in respectivecurrent paths by the pulses, and FIG. 2E depicts the angular location ofthe enabled switch during each successive pulse of FIG. 2C in accordancewith one example. In FIG. 2E, the beam impact point 145 may be heldconstant for a selected number of pulses during which different contactrods 160 are selected. Thereafter, the X- and Y-axis stages 150, 155 mayoperate to shift the location of the impact point 145, and the proceduremay then be repeated. One example is depicted in FIG. 1D, depicting theperipheral contact belt 115 of the wafer 100 and its points of contactwith the contact rods 160. FIG. 1D further shows an example of differentbeam impact points 145-1, 145-2 on the wafer 100, with different contactrods 160 being selected for each to establish different current paths.In one embodiment, for example, one beam impact point 145-1 may be at aninner radius, while another beam impact point 145-2 may be at an outerradius. Each beam impact point may receive a number of beam pulses, witha different ones of the contact rods 160 being enabled at differenttimes to establish a different current path. In this manner, a model ofthe radial distribution of sheet resistance may be constructed by thecomputer 180 (FIG. 1A) so that radial non-uniformity in sheet resistancedistribution may be deduced by the computer 180. Such informationconstructed by the computer 180 may be furnished as a feed forwardcorrection to an electroplating process to be performed on the wafer100.

FIGS. 3A and 3B depict a modification of the embodiment of FIGS. 1A and1B, in which the beam source 140 (i.e., the electron beam source) isreplaced by a high energy (e.g., ultraviolet) light beam source 140′,which generates a light beam 142′. The optical frequency of the lightbeam 142′ in one embodiment corresponds to a photon energy exceeding thework function of the metal layer 105. A small overhead anode electrode190 provided within the vacuum enclosure 172 to capture electronsemitted from the metal layer 105 due to interaction with the light beam142′. In one embodiment, the anode electrode 190 is mechanically coupledto the light beam source 140′, and follows movements of the light beamsource 140′ controlled by the X-axis stage 150 and Y-axis stage 155. Inone embodiment, the diameter of the anode electrode 190 is sufficientlysmall to avoid interference with movement of the light beam source 140′by the X-axis stage 150 and the Y-axis stage 155, permitting movement ofthe beam impact point 145 across the diameter of the wafer 100. Theanode electrode 190 is connected to the positive terminal of a D.C.voltage source 192, whose negative terminal is connected to ground. Thewafer 100 acts as a cathode in this embodiment. As depicted in FIG. 3C,the anode electrode 190 has a central hole 194 through which the lightbeam 142′ passes. The light beam source 140′ in the embodiment of FIGS.3A-3C may be either a pulsed beam source or a continuous beam source.The embodiment of FIGS. 3A-3C can include the other features of theembodiment of FIGS. 1A-1C. If the light beam source 140′ is a pulsedbeam source, then the desired measurement of a material characteristic(e.g., sheet resistance) is inferred from the behavior of the output ofthe electrical sensor 135 during successive pulsed beam off times, inthe manner described above with reference to FIGS. 1A-1C. If the lightbeam source 140′ is a continuous beam source, then the desiredmeasurement of a material characteristic is inferred from the magnitudeof the output of the electrical sensor 135, in the manner describedabove with reference to FIGS. 1A-1C.

Feed Forward Control of Electroplating Process:

In one embodiment, the computer 180 of FIG. 1A may be programmed to usemany successive measurements of a material characteristic (e.g., sheetresistance) in the metal layer 105 along many different paths, toconstruct a distribution model 181 representing distribution of thematerial characteristic in the metal layer 105 across the wafer surface.In the remainder of this detailed description, the materialcharacteristic is referred to as sheet resistance, although othermaterial characteristics of the metal layer 105 may be measured instead.For example, the thickness of the metal layer 105 may be inferred fromthe sheet resistance. Thus, the distribution model 181 may representdistribution of any suitable characteristic of the metal layer, such assheet resistance, thickness, or other characteristic. The distributionmodel 181 constructed by the computer 180 may be used by the computer180 to produce a signal representing feed forward correction to anelectroplating process in which a thick metal (copper) layer is formedover the thin metal layer 105 on wafer 100. The feed forward correctioncompensates for non-uniformity represented in the distribution model181. For example, measurements of sheet resistance distribution in themetal layer 105 taken at the conclusion of the PVD process for barrierand seed layer deposition may be used to compensate for the measurednon-uniformities during an electroplating process performed later on thesame wafer.

A system incorporating such feed forward process control is depicted inFIG. 4. A physical vapor deposition (PVD) reactor 200 is employed todeposit the thin (e.g., 50-200 Angstroms) barrier and copper seed layersreferred to above. An electroplating tool or electroplating reactor 210is employed to form a thick copper layer over the thin copper seedlayer. An energy beam measurement tool 220 of FIG. 4 embodies the energybeam measuring apparatus of FIGS. 1A-1C. The energy beam measurementtool 220 may be included in the PVD reactor 200 (to measure a wafer uponcompletion of either the barrier and/or seed layer deposition step).Alternatively, the energy beam measurement tool 220 may be included inthe electroplating reactor 210 in order to measure incoming wafers priorto performing the electroplating step. A wafer transfer robot 250transports the wafers from the PVD reactor 200 to the electroplatingreactor 210. The energy beam measurement tool 220 provides to theelectroplating reactor 210 a feed forward signal representing orcontaining information regarding non-uniformity in the barrier and/orseed layer. The electroplating reactor 210 responds to the feed forwardsignal by adjusting radial distribution of the electroplating depositionrate.

An embodiment of an electroplating reactor that is capable of adjustingradial distribution of deposition rate is depicted in FIGS. 5A and 5B.The electroplating reactor of FIGS. 5A and 5B is used to form a thickmetal (copper) layer over the metal layer 105 referred to previouslywith reference to FIG. 1C. The electro-plating tool of FIGS. 5A and 5Bincludes a bath container 300 filled with a solution 305 such as acopper-containing liquid electrolytic solution (e.g., copper sulfate).Plural (e.g., at least two) anode rings 310, 315 immersed in thesolution 305 are driven by respective anode voltage sources 320, 325.The anode rings 310, 315 may be concentric. The front side (only) of thewafer 100 (i.e., the side on which the integrated circuit structures areformed) is immersed in the solution 305 facing the anode rings 310, 315and spaced from them. The back side of the wafer 100 is outside or abovethe solution 305. A cathode ring 330 contacts the peripheral contactbelt 115 on the front side of the wafer 100 to provide a completecircuit for the electro-plating current. In one embodiment, the cathodering 330 supports the wafer 100, and may be grounded (or connected to asuitable return potential referenced to the anode voltage sources 320,325). A controller 360 governs a voltage ratio between the anode voltagesources 320, 325 to control the radial distribution of the depositionrate of the electroplating process.

In one embodiment, the controller 360 receives a feed forward signal 362from the energy beam measurement tool 220 shown in FIG. 4, which is ofthe type described with reference to FIG. 1A or 3A. The feed forwardsignal 362 is derived from the distribution model 181 of FIG. 1A or 3A,and represents the distribution of a chosen characteristic (e.g., sheetresistance or thickness) of the underlying barrier and seed layers. Thecontroller 360 determines from the feed forward signal 362 a desiredvoltage ratio between the anode voltage sources 320, 325 to correct thenon-uniformity. The controller 360 may be programmed to determine theoptimum voltage ratio based upon non-uniformity in a measureddistribution of sheet resistance. For example, an edge-highnon-uniformity in radial distribution of sheet resistance may becompensated by a distribution of anode source voltages that iscenter-high (or edge-low). Conversely, a center-high non-uniformity inradial distribution of sheet resistance may be compensated by adistribution of anode voltages that is edge-high (or center-low).

In-Situ Feedback Control of the Electroplating Process:

FIG. 6A illustrates a modification of the embodiment of FIGS. 5A and 5B,in which feedback control of the electroplating process is provided inreal time during the electroplating process based upon measurementscontinuously performed on the wafer. In the embodiment of FIG. 6A, thewafer 100 is held by the cathode ring 330 on the surface of the solution305, so that only the front side of the wafer 100 (e.g., the side of thewafer 100 on which the integrated circuit structures are formed) is inthe solution 305, while the back side of the wafer 100 is above (notimmersed in) the solution 305. An array of plural sensors 400, such asplural eddy current loss measuring sensors (hereinafter referred to aseddy current sensors) are placed on the back side of the wafer 100. Inone embodiment, the sensors 400 include a sensor 400-1 at a radiallyinner location and a sensor 400-2 at a radially outer location on thewafer back side. Each of the sensors 400 in the array is referred togenerically as a thickness sensor. As employed in this specification,the term “thickness sensor” refers to any sensor, such as an eddycurrent sensor, capable of sensing a parameter affecting deposition rateor thickness in a metal plating process. This parameter may be sheetresistance, resistivity, conductance, conductivity or thickness itself.The thickness sensor has an output indicative of a measured value of theparameter.

A computer 180-1 periodically samples the output signals from the eddycurrent sensors 400-1 and 400-2 and deduces from the latest sample ofthose signals an instantaneous radial distribution of thickness of thedeposited metal films. The computer 180-1 constructs, from theinstantaneous radial distribution, a corrective feedback signal andsends it to the controller 360. This corrective signal represents achange in the voltage ratio between the anode voltage sources 320, 325that compensates for the non-uniformity in radial thickness distributiondeduced according to the latest sample.

Referring again to FIG. 6A, wafer handling apparatus 410, is disposedover the wafer 100 and restricts the space above the array of sensors400 to a small vertical clearance. It is difficult to fit a conventionaleddy current sensor into this restricted space. To overcome thisproblem, a very thin eddy current sensor employing a planar inductor isprovided. Its structure may be implemented in each individual eddycurrent sensor 400-1, 400-2, etc., of the array of sensors 400. Theplanar inductor is depicted in FIGS. 6B and 6C as an elongate conductor440 supported on a dielectric layer or substrate 441. The elongateconductor 440 will be referred to herein as the planar inductor 440. Theplanar inductor 440 forms a serpentine pattern defining rectangularserpentine loops 440 a, 440 b, 440 c, etc. Each serpentine loop 400 a,400 b, 400 c, etc., is depicted as having a rectangular shape, althoughother shapes may be employed. The drawing of FIG. 6B depicts the planarinductor 440 as including ten serpentine loops 400 a through 400 j,although another number of serpentine loops may be employed.

In the embodiment of FIG. 6A, the wafer 100 is inverted to face down,and the front side of the wafer 100 is immersed in the solution 305while the wafer back side is above the liquid surface of the solution305. The eddy current sensor including the planar inductor 440 overliesthe back side of the wafer 100 while the metal layer 105 is on the front(immersed) side of the wafer 100. In other uses in which the wafer isnot inverted, the eddy current sensor may directly overlie the metallayer 105 on the wafer front side.

FIG. 6C depicts the magnetic fields B produced at successive portions ofthe planar inductor 440. FIG. 6D depicts the instantaneous eddy currentloops M produced in the metal layer 105 at the respective serpentineloops 440 a, 440 b, 440 c, etc. The planar inductor 440 of FIG. 6Dproduces plural eddy currents M distributed along its path in theunderlying metal layer 105. In contrast, a conventional eddy currentsensor employs a single multi-turn coil that produces a single eddycurrent loop in the underlying metal layer 105. The plural eddy currentsM produced by the planar inductor 440 ensure a response to changes inthickness that is at least as great as that of the single multi-turncoil of a conventional sensor.

In the embodiment of FIG. 6E, the planar inductor 440 has a pair of endterminals 440-1, 440-2 which are adjacent one another. As shown in FIG.6E, the planar inductor 440 forms a large circuit consisting of manyserpentine loops 400 a, 400 b, 400 c, etc., distributed along oppositedirections from the pair of end terminals 440-1, 440-2. In theembodiment of FIG. 6E, the large circuit of the planar inductor 440includes a pair of opposing serpentine row patterns 442-1 and 442-2parallel to one another, to complete a circuit between the end terminals440-1 and 440-2. While the row patterns 442-1, 442-2 of FIG. 6E form arectangular sensor shape or footprint, the planar inductor 440 may bemodified to form another sensor shape, which may be circular, ellipticalor arcuate, for example.

As depicted in FIGS. 6E and 6F, the planar eddy current sensor includesa sensor circuit block 411 connected to the planar inductor 440. Thesensor circuit block 411 may be thin as the planar inductor, and may beimplemented as integrated circuitry. The sensor circuit block 411includes an RF oscillator 542 driving the planar inductor 440, animpedance sensing circuit 544 coupled to the planar inductor 440, and acapacitance sensor 546. Both the impedance sensing circuit 544 and thecapacitance sensor 546 have their outputs coupled to the computer 180-1via a cable 460. Changes in thickness in the underlying metal layer 105(FIG. 6A) change the RF impedance of the planar inductor 440, which isdetected by the impedance sensing circuit 544, and may be translated bythe computer 180-1 to a thickness measurement. The capacitance sensor546 is a displacement sensor that monitors the lift-off distance orheight of the planar inductor 440 above the metal layer 105 (FIG. 6A)under measurement. In one embodiment, the measured lift-off distance isused by the computer 180-1 to calibrate each measurement. In oneembodiment, the bottom of the capacitance sensor 546 may be co-planarwith the plane of the planar inductor 440.

FIG. 6F depicts the sensor of FIG. 6E in elevation, showing the planarinductor 440 as a conductive thin film, and the dielectric layer orsubstrate 441 underlying the planar inductor 440. Optionally, aninsulator layer 444 may cover the top of the planar inductor 440.

FIG. 6G depicts a modification of the embodiment of FIGS. 6E-6F, inwhich half-circular ferrite cores 450 a, 450 b, 450 c, etc., overlierespective sections of the planar inductor 440. The ferrite cores 450 a,450 b, 450 c, etc., confine the local magnetic fields producing therespective eddy current loops, hence producing a more confined eddycurrent and enhancing signal output.

FIGS. 6H and 61 depict an embodiment in which the individual ferritecores 450 a, 450 b, 450 c, etc., of FIG. 6D are replaced by anintegrated ferrite core layer 450 providing continuous coverage of theplanar inductor 440 along its entire length.

FIG. 6J depicts and embodiment in which the conductor of the planarinductor 440 is wound in a rectangular spiral.

Some embodiments may employ more than one eddy current sensor at eachradial location. For example, plural eddy current sensors 400 may bedisposed in circular arrays at different radial locations, as depictedin FIG. 7A. Further, the annular anodes may be replaced by circulararrays of discrete anodes 317, as depicted in FIG. 7A, whose voltagesare individually controlled by the controller 360. In FIG. 7A, thediscrete anodes 317 are submerged in the solution 305, while the arrayof eddy current sensors 400 are on the wafer back side and above thesolution 305. In another embodiment depicted in FIG. 7B, the array ofeddy current sensors 400 may be a two-dimensional array, and the anodesmay be provided as a two dimensional array of discrete anodes 317 whosevoltages are individually controlled by the controller 360. The computer180-1 may be coupled to each individual sensor 400 in the array. Thiscan enable the computer 180-1 to compensate for both radialnon-uniformities and azimuthal non-uniformities in thicknessdistribution.

In one embodiment, the distribution model 181 of the barrier and seedlayers determined for a given wafer by the energy beam measurement toolof FIG. 1A (or FIG. 3A) is used to determine an optimum initialdistribution of anode voltages prior to the beginning of theelectroplating process performed on the same wafer by the electroplatingreactor of FIG. 6A. In one embodiment, the controller 360 (or thecomputer 180-1) of FIG. 6A receives a feed forward signal 362 from thebeam measurement tool of FIG. 1A or 3A. The feed forward signal 362 isderived from the distribution model 181 (FIG. 1A or 3A), and representsthe distribution or distribution non-uniformity of a chosencharacteristic (e.g., sheet resistance or thickness) of the underlyingbarrier and seed layers. The controller 360 (or the computer 180-1)determines from the feed forward signal 362 a desired voltage ratiobetween the anode voltage sources 320, 325 to correct thenon-uniformity. The controller 360 (or the computer 180-1) may beprogrammed to determine the optimum voltage ratio based uponnon-uniformity in a measured distribution of sheet resistance, forexample. After the electroplating process has begun, process controlpasses to the feedback control loop of FIG. 6A described above, whichuses the thickness measurements taken in real time by the array of eddycurrent sensors 400.

In one version of the embodiment of FIG. 6A, each eddy current sensor400 may be pressed against the back side of the wafer 100, providing afixed displacement during plating between the eddy current sensor andthe plating interface at the front (device) side of the wafer. (In sucha case, the capacitance sensor 546 of FIG. 6E may be eliminated, tosimplify the hardware.) This feature is advantageous because the eddycurrent signal is sensitive to the distance of the sensor from the waferbeing measured. Keeping this displacement constant during platingfacilitates precise sheet resistance and thickness measurements.

Sensors with different resolutions or sensitivities may be placed atoptimal locations on the wafer. Early-stage plating at the wafer edge isparticularly critical, for example, and a very sensitive eddy currentsensor may be placed near the edge of the wafer. In the embodiment ofFIG. 6A, the plural eddy current sensors 400 may be selected fordifferent thickness ranges to provide a combination of optimalsensitivity and optimal range for different thicknesses of the metallayer 105. For example, in one embodiment, a first one of the eddycurrent sensors 400 may be selected to provide high sensitivity for thevery thin Cu films that are encountered at the start of plating (e.g.,less than 150 Angstroms thick). This high-sensitivity sensor saturatesas the film gets thicker. A second one of the sensors 400 is selectedwith a lower resolution and larger range, to measure thicker films(e.g., 150-2000 Angstroms thick) with reduced absolute resolution, so asto complement the first (high-sensitivity) sensor.

Multiple eddy current sensors 400 with successive overlapping ranges maybe provided in a further embodiment, as follows:

first sensor range: 0-150 Angstroms

second sensor range: 125-275 Angstroms

third sensor range: 250-750 Angstroms

fourth sensor range: 500-1500 Angstroms

fifth sensor range: 1200-2000 Angstroms.

The in situ closed loop feedback control described above with referenceto FIG. 6A enables uniform plating thickness across the wafer.Well-controlled temporal and spatial plating rates are also critical toavoid formation of voids in a damascene structure as it is filled duringplating. Variations or non-uniformities in thickness distribution can becorrected after the electroplating process during chemical mechanicalpolishing (CMP), but voids are unrecoverable sources of yield loss.

A sequence over time of successive thickness measurements by the arrayof eddy current sensors 400 may be used by the computer 180-1 to obtaina relative measure of change in sheet-resistance and thickness. From atime-series of measurements in the embodiment of FIG. 6A, the computer180-1 can provide the user with an estimate of plating rates atdifferent times and relative plating rates at different locations aroundthe wafer without precise calibration of absolute sensor levels. Thefull time series of eddy current sensor outputs can also be fit to astraight line (or other suitable function) to provide a much moreprecise measure of rate of sheet resistance change than one or twodiscrete measurement points.

The measurement through the wafer back side in the embodiment of FIG. 6Acan be carried out using sensors other than eddy current sensors.Acoustic sensors could, for example, be used to detect voiding in aparticular area of the wafer.

Routing signals from the sensors 400 on the back side of the wafer tothe computer 180-1 may be carried out while the wafer is rotated in thesolution 305 by the wafer handling apparatus 410 above the wafer.Signals are passed through a rotating member of the wafer handlingapparatus 410 in such a case. An alternative mode uses wireless signaltransmission, which is possible in this environment in which most of theenclosure is plastic.

Rapid Acquisition of Thickness Distribution after Electroplating:

After completion of the electroplating process, a final measurement ofmetal thickness and spatial distribution of the thickness is needed.Such a final measurement facilitates, for example, monitoring theintegrity of the contact ring 130 and of the peripheral contact belt115. This latter measurement entails measuring metal thickness along thelength of the peripheral contact belt 115 following completion of theelectroplating process. Conventional use of an eddy current sensor toperform such measurements involves the taking of many discrete samplesat successive locations at which the sensor is temporarily heldstationary, which is time consuming and entails a significant expense.

This problem is solved in the embodiment of FIGS. 8 and 9, in which acontinuous measurement of electroplated thickness distribution isperformed by spinning the wafer at a high rate under an eddy currentsensor suspended over the wafer front side, and changing the radialposition of the eddy current sensor. In one embodiment, the eddy currentsensor output is constantly sampled and correlated with successiveradial and azimuthal positions of the eddy current sensor relative tothe wafer, to rapidly acquire an accurate spatial image of thicknessdistribution.

Referring now to FIGS. 8, a measurement tool 500 is capable of rapidlymeasuring thickness distribution across a wafer surface. The measurementtool 500 includes a vacuum chuck 505 for holding the wafer 100, aspinning stage 510 supporting the vacuum chuck 505 and a notch detector512 stationed over or adjacent the wafer edge for detecting a notch 101in the wafer edge (to measure the wafer rotational position). Themeasurement tool 500 further includes a rotatable vertical post 515, arotation motor 520 coupled to the vertical post 515, a horizontal swingarm 525, a piezo electric vertical position actuator or transducer 530and an eddy current sensor 535 mounted on the transducer 530. In amodification depicted in FIG. 8A, there is an array of plural eddycurrent sensors 535 a through 535 f mounted on the transducer 530, aswill be described later herein.

In one embodiment, the vertical post 515 includes an actuator 515 a toraise or lower the arm 525 and the eddy current sensor 535 together in acoarse vertical motion, whereas the transducer 530 enables a finervertical motion for a better control of the lift-off distance Z betweenthe eddy current sensor 535 and the wafer surface. As depicted in FIG.9, the wafer 100 rotates with the spinning stage 510 while the eddycurrent sensor 535 is moved radially over the wafer 100 along an arcuatepath 537. A continuous measurement signal from the eddy current sensor535 is recorded (or successively sampled) by a computer 180-2 as afunction of location on the wafer. This location is constantly shiftedover time, in the manner depicted in FIG. 9. The computer 180-2 governsa controller 560 that controls the rotation motor 520 and the spinningstage 510. An output signal of the notch detector 512 is coupled to thecomputer 180-2. The location of the measurement is inferred by thecomputer 180-2 from the instantaneous angular position of the swing arm525, the rotational speed of the spinning stage 510 and the elapsed timesince the notch detector 512 last detected the notch 101. Therecordation of such a continuous measurement is depicted in the graph ofFIG. 10 for a fixed radial position of the eddy current sensor 535. Thevertical axis of FIG. 10 represents measured thickness while thehorizontal axis represents azimuthal (angular) position around the axisof the wafer. The allowable maximum and minimum thickness limits aresuperimposed in the graph, from which the computer 180-2 identifiesout-of-tolerance conditions. Different graphs like that of FIG. 10 areproduced for different radial positions of the eddy current sensor 535.

FIG. 11 depicts an embodiment of the eddy current sensor 535 having ashaped measuring zone, for use in any of the embodiments of FIGS. 8-9and 12-14. The eddy current sensor of FIG. 11 includes a coil 540, an RFoscillator 542 driving the coil 540, an impedance sensing circuit 544coupled to the coil 540, and a computer 180-3 connected to the output ofthe impedance sensing circuit 544. Changes in underlying materialthickness change the RF impedance of the coil 540, which are detected bythe impedance sensing circuit 544, and correlated by the computer 180-3to a thickness measurement. A displacement sensor or capacitance sensor546 monitors the lift-off distance or height of the coil 540 above thewafer surface. In one embodiment, the measured lift-off distance is usedby the computer 180-3 to calibrate each measurement accordingly. In oneembodiment, the bottom of the capacitance sensor 546 may be co-planarwith the bottom of the coil 540. In addition, or alternatively, thelift-off distance measured by the capacitance sensor 546 is used toconstruct a corrective feedback signal to the transducer 530 in afeedback loop to maintain a constant lift-off distance Z. In oneembodiment, the corrective feedback signal is constructed by thecomputer 180-3 based upon the output of the capacitance sensor 546. Theeffect of the offset position between the eddy-current and capacitancesensors can be eliminated by synchronizing the motion and the dataacquisition. The shape of the eddy current field, determined by theshape or footprint of the coil 540, can also be optimized for aparticular geometry. A long thin or even a curved sensor can be employedto provide an average reading for a particular region of the wafer suchas the periphery or circumference of the wafer. For example, FIG. 11depicts the coil 540 having an arcuate geometry, and has an arcuatefootprint or measuring zone defined by the shape of the coil 540. In theillustrated embodiment, the boundary of the coil 540 in its bottom planeincludes a pair of generally congruent or parallel arcs 540 a, 540 bdisplaced from one another by a distance W corresponding to the width ofthe sensor footprint. In one embodiment, the parallel arcs 540 a, 540 bmay each have a radius centered at the axis of symmetry of the wafer100. While the coil 540 in the illustrated embodiment of FIG. 11 ishelically wound, in another embodiment the coil 540 may be replaced bythe planar inductor 440 of FIGS. 6B-6I described above.

While this specification refers to several computers 180, 180-1, 180-2and 180-3, it is understood some or all of the computers referred to maybe implemented as a single computer.

FIG. 12 depicts a modification of the embodiment of FIG. 9 employing agantry 570 to control movement of the eddy current sensor 535 along oneaxis. FIG. 13 depicts a modification of the embodiment of FIG. 9 inwhich the swing arm 525 is retractable to adjust the radial position ofthe eddy current sensor 535. FIG. 14 depicts a modification of theembodiment of FIG. 9, in which the spinning stage 510 providestranslational movement of the wafer 100 along either (or both) X- andY-axes or along a radial direction and vertical motion to adjustlift-off distance Z, in addition to spinning the wafer 100, while theeddy current sensor 535 is stationary on a fixed mount 536.

The method performed by the apparatus of any of FIGS. 8-14 has theadvantage of providing a more complete picture of the thicknessvariation along a particular radius. It is well-suited to detectthickness excursions along the edge of the wafer. Small spatialnon-uniformities are not missed because all positions along the edge arecontinuously measured. It is also a fast measurement. The wafer can berotated at more than 1 revolution per second. For example, in 1 second,several snapshots of the thickness profile at a certain radius arecollected. The multiple snapshots can be averaged to reduce noise, andto amplify small thickness non-uniformity. Discrete-point informationcan be extracted by synchronizing the motion and the data acquisitionwith the aid of the notch detector to orient the wafer.

Faster data acquisition can be performed by attaching multiple (e.g. N)eddy current sensors, such as the sensors 535 a-535 f of FIG. 8A, on thetransducer 530.

The sensors 535 a-535 f may be arranged along an imaginary straight lineextending radially with respect to the axis of the vertical post 515.Alternatively, the sensors 535 a-535 f may be mounted in this manner onthe transducer 530 in the embodiments of FIGS. 12 and 13 or on the fixedmount 536 of FIG. 14. The sensors 535 a-535 f may form or be included inan array of N sensors where N is an integer. The N sensors including thesensors 535 a-535 f sweep multiple radii simultaneously in a singlewafer rotation, collecting N times as much data as a single sensor wouldcollect. Signal matching from the N sensors can be provided by carefulcalibration of each sensor using a common set of reference films 575with known thickness. In one embodiment, the set of reference films 575may be mounted on a tray 580 located near the side or perimeter of thevacuum chuck 505, outside the diameter (e.g., 300 mm) of the wafer 100(as depicted in FIG. 8A). Periodic re-calibration of the sensors 535a-535 f is conducted as needed.

Improvements in the thermal stability of the measurement apparatus ofFIG. 8 can be achieved by including a temperature sensor 591 (e.g., athermocouple, or resistance temperature detector) inside the eddycurrent sensor 535, and including a temperature sensor 592 in the vacuumchuck 505 to measure the wafer temperature. The information from thetemperature sensors 591 and 592 can be used by the computer 180-2 tocompensate for any thermal drift in the coil and the wafer that canaffect the eddy-current signal. The signal compensation is possible whenthe signal is pre-calibrated vs. temperature during the initial set-up.

For measurements involving a blanket conductive film deposited on top ofanother conductive film or patterned conductive structures, theunderlying layer/pattern can induce appreciable eddy-current signal thatmay interfere with the signal of interest from the top-most layer. Insuch a situation, two measurements can be taken: the first one is beforethe deposition of the layer of interest, the second one is after thedeposition. The first measurement will map the signal variation from theunderlying layer/pattern throughout the wafer, which can then besubtracted from the second measurement, such that the true thicknessvariation of the top layer of interest can be accurately measured.

The high speed thickness measurement tool of FIGS. 8 and 9 may providefeedback correction to the electroplating reactor of FIG. 5A or 6A.Specifically, the computer 180-2 of FIG. 8 or FIG. 8A may be programmedto determine non-uniformity in the thickness distribution after theelectroplating process is completed, and to generate a feedbackcorrection signal 363 indicated in FIG. 6A. In the electroplatingreactor of FIG. 5A or 6A, the controller 360 may be programmed to adjustthe distribution of anode voltages to compensate for the non-uniformityin thickness distribution. The computer 180-2 may provide thiscorrection signal either to the controller 360, as indicated in FIG. 6A,or to the electroplating reactor computer 180-1, for example.

Integrated System with Feed forward and Feedback Control:

FIG. 15 depicts an integrated system capable of performing feed forwardcontrol and feedback control of the electroplating process. The systemincludes the PVD reactor 200 of FIG. 4, an energy beam measurement tool220 of the type described with respect to FIG. 1A or 3A, anelectroplating reactor 210 of the type described above with reference toFIG. 6A having in situ feedback, and a measurement tool 500 of the typedescribed above with reference to FIGS. 8-14. As depicted in FIG. 15,the energy beam measurement tool 220 may be in either one of twolocations (e.g., in the PVD reactor 200 or in the electroplating reactor210). In one embodiment, a wafer transfer robot 251 provides wafertransfer from the PVD reactor 200 to the electroplating reactor 210, andfrom the electroplating reactor 210 to the measurement tool 500. FIG. 16depicts a process performed by the system of FIG. 15 employing feedforward control and feedback control.

The energy beam measurement tool 220 provides information to theelectroplating reactor 210 defining non-uniformities in an incomingwafer prior to the beginning of the electroplating process. Thisinformation serves as feed forward control information, which theelectroplating reactor 210 uses to establish an initial distribution ofanode supply voltages to the array of electroplating anodes (e.g., theanode rings 310, 315 of FIG. 6A). The electroplating reactor 210 of FIG.15 includes an array of eddy current sensors 400 providing real-timefeedback of thickness distribution information used to control thedistribution of anode supply voltages. The measurement tool 500 may beprovided as a part of the electroplating reactor 210 (or separately),and measures a metal thickness distribution on a wafer upon completionof the electroplating process. The thickness distribution measurementsby the measurement tool 500 for one wafer may (in one embodiment) beprovided as feedback information for the next wafer to theelectroplating reactor 210. The computer 180-1 may use this feedbackinformation as a one-time correction to refine the anode supply voltagedistribution of the electroplating reactor 210.

FIG. 16 illustrates one embodiment of a method in the system of FIG. 15.FIGS. 17A-17D depict changes in thin film structure corresponding tocertain steps of FIG. 16. Referring now to FIG. 16, an initial step,depicted in FIG. 17A, is to form a trench 700 in a semiconductor layeror substrate 705 (block 600 of FIG. 16). The wafer is then transferredto the PVD reactor 200, in which a barrier layer 710 is deposited (block610 of FIG. 16) as depicted in FIG. 17B. Optionally, the energy beammeasurement tool 220 may be used to perform an initial measurement ofsheet resistance distribution in the barrier layer 710 (block 615 ofFIG. 16). Thereafter, a copper seed layer 715 is deposited over thebarrier layer (block 620 of FIG. 16) as depicted in FIG. 17C. At thispoint, a final energy beam measurement of sheet resistance distributionis performed on the wafer (block 630 of FIG. 16), and this distributionis provided to the electroplating reactor 210 as feed forward control(block 640 of FIG. 16).

At or prior to the beginning of the electroplating process, the anodevoltage distribution within the electroplating reactor 210 isinitialized or set in accordance with feed forward information from theenergy beam measurement tool 220. If this information indicates aparticular non-uniformity in sheet resistance distribution, then theanode voltage distribution is initially configured to best compensatethe non-uniformity. For example, a center-high sheet resistancedistribution is a predictor of a center-high electroplating growth ratedistribution. Therefore, such a non-uniformity may be compensated by acenter-low anode voltage distribution in the electroplating reactor 210.Likewise, a center-low sheet resistance distribution measurement by theenergy beam measurement tool 220 is compensated by a center-high anodevoltage distribution in the electroplating reactor 210. The feed forwardfunction of block 640 may be implemented by the computer 180-1 toachieve the above-described compensation, and the computer 180-1 may beprogrammed accordingly.

During the electroplating process (block 650 of FIG. 16), a thick copperlayer 730 is deposited over the copper seed layer 715 and fills thetrench 700, as depicted in FIG. 17D. The anode voltage distribution inthe electroplating reactor 210 is adjusted in accordance with real-timefeedback from the array of eddy current sensors 400 on the wafer backside (block 660 of FIG. 16). Such real time feedback indicates either acenter-high or a center-low thickness distribution at each instant(sample) in time, which is compensated by a center-low or a center-highanode voltage distribution. The computer 180-1 may be programmed toprovide such compensation in response to the feedback from the eddycurrent sensor array.

After completion of the electroplating process of a given wafer, themeasurement tool 500 measures the thickness distribution across thewafer (block 670 of FIG. 16). This distribution, in one embodiment, isfed back to the electroplating reactor 210 as feedback to refine theanode voltage distribution for the next wafer. Thus, for example, acenter-high thickness distribution measurement by the measurement tool500 is compensated by adjusting the anode voltage distribution in theelectroplating reactor 210 to be more center-low. Likewise, a center-lowthickness distribution measurement by the measurement tool 500 iscompensated by adjusting the anode voltage distribution in theelectroplating reactor 210 to be more center-high. Such compensation maybe controlled by the computer 181-1, which may be programmedaccordingly.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

What is claimed is:
 1. An electroplating reactor, comprising: anelectroplating bath configured to hold an electro-plating solutionhaving a liquid top surface; a ring cathode in said bath and located tocontact a workpiece such that a front side of the workpiece is immersedin said solution below said liquid top surface and a back side of theworkpiece is above said liquid top surface; plural anodes in said bathbelow said ring cathode, and plural anode voltage sources coupled tosaid plural anodes; plural thickness sensors at spatially separatelocations on the back side of the workpiece; and a computer coupled tooutputs from said plural thickness sensors and coupled to said pluralanode voltage sources.
 2. The electroplating reactor of claim 1 whereinsaid plural thickness sensors comprise plural eddy current sensors. 3.The electroplating reactor of claim 1 wherein said computer isprogrammed to infer from the outputs of said sensors a spatialdistribution of thickness on said front side of said workpiece, and tocontrol a distribution of voltages from said anode voltage sources tocompensate for non-uniformity in said spatial distribution.
 4. Theelectroplating reactor of claim 1 wherein said anodes compriseconcentric rings.
 5. The electroplating reactor of claim 1 wherein saidplural thickness sensors are located at different radial locations. 6.The electroplating reactor of claim 1 wherein said anodes comprise atwo-dimensional array of discrete anodes.
 7. The electroplating reactorof claim 1 wherein said plural thickness sensors are arranged in atwo-dimensional array.
 8. The electroplating reactor of claim 1 whereinat least one of said plural thickness sensors has an arcuate footprint.9. The electroplating reactor of claim 8 wherein said arcuate footprintcorresponds to an arc having a radius extending from an axis of symmetryof said workpiece.
 10. The electroplating reactor of claim 2 wherein atleast one of said plural eddy current sensors comprises: a planarinductor formed in a thin planar layer comprising plural co-planarconductor loops connected in series and distributed in a plane; an RFoscillator coupled to said inductor; and an impedance-sensing circuitcoupled to said planar inductor.
 11. The electroplating reactor of claim10 wherein said conductor loops are formed as an elongate conductor,said planar inductor further comprising a dielectric support layerunderlying said thin planar layer.
 12. The electroplating reactor ofclaim 11 wherein said plane of said conductor loops faces a plane ofsaid back side of said workpiece.
 13. The electroplating reactor ofclaim 10 wherein said conductor loops define a serpentine shape.
 14. Theelectroplating reactor of claim 13 wherein each one of said conductorloops is shaped as a rectangular loop having three straight sides andone open side.
 15. The electroplating reactor of claim 13 wherein saidplural conductor loops provide a closed path of said plural conductiveloops.
 16. A system for electroplating a workpiece having a previouslydeposited metal film, comprising: an electroplating bath configured tohold an electroplating solution having a liquid top surface; a ringcathode in said bath and located to contact a workpiece such that afront side of the workpiece is immersed in said solution below saidliquid top surface and a back side of the workpiece is above said liquidtop surface; plural anodes immersed in said bath below said ringcathode, and plural anode voltage sources coupled to said plural anodes;plural thickness sensors at spatially separate locations on the backside of the workpiece; a first computer coupled to outputs from saidplural thickness sensors and coupled to said plural anode voltagesources; and a measurement tool comprising an energy beam sourcedirected to a workpiece to be measured, an electrical sensor coupled tothe workpiece to be measured and a second computer programmed to infer aspatial distribution of an electrical parameter from observations ofsuccessive outputs of said sensor for different current paths in saidmetal film, said second computer coupled to said first computer.
 17. Thesystem of claim 16 wherein: said first computer is programmed to adjustan initial distribution of the voltages of said anode voltage suppliesin accordance with said spatial distribution of said electricalparameter.
 18. The system of claim 17 wherein said first computer isprogrammed to adjust said initial distribution of said voltages of saidanode voltage supplies to compensate for non-uniformity in said spatialdistribution of said electrical parameter.
 19. A method of depositing ametal layer on a workpiece by electroplating, comprising: submersing aworkpiece in a bath containing an electrolyte solution having a liquidtop surface so as to immerse a front side of the workpiece in thesolution and leave a back side of the workpiece above the liquid topsurface; electrically contacting said front side of said workpiece;providing plural separate anodes in said bath and respective anodevoltage sources coupled to said anodes; affixing plural thicknesssensors to said back side of said workpiece at different locations;inferring from outputs of said thickness sensors a spatial distributionof thickness on said front side of said workpiece; and adjusting adistribution of voltages among said anode voltage sources to compensatefor non-uniformity in said spatial distribution.
 20. The method of claim19 wherein said anodes are disposed at respective radial locations, andwherein said adjusting comprises one of: (a) providing a center-highdistribution of said anode voltages whenever said spatial distributionis center-low, (b) providing a center-low distribution of said anodevoltages whenever said spatial distribution is center-high.